BEIRUT The U.S.-backed Syria Democratic Forces (SDF) alliance has crossed into Deir al-Zor province for the first time as part of an offensive against Islamic State, a Kurdish military source said on Tuesday.The advance into the province, which is almost entirely under the control of the ultra-hardline group, is part of an operation to encircle and ultimately capture its de facto Syrian capital of Raqqa in the north of the country. One aim of the campaign is to cut IS's supply lines from Raqqa to Deir al-Zor province."Military operations of the SDF are now taking place within the provincial boundaries of Deir al-Zor, from the north - so, via southern Hasaka (province)," the Kurdish military official told Reuters.The SDF, which includes the Kurdish YPG militia and Arab fighting groups, captured some 15 villages from Islamic State militants in their incursion into the province, the source added. The source did not specify when this had taken place.Islamic State controls all of Deir al-Zor province apart from a Syrian government-held enclave in Deir al-Zor city and a nearby military air base.A number of different groups in Syria's multi-sided conflict are fighting separate battles against Islamic State, which has made an enemy of all sides.

Syria's army and its allies, backed by Russia, have been fighting back Islamic State assaults in Deir al-Zor city and have engaged in clashes with the group further west.HUMANITARIAN CONCERNS
The SDF advance into Deir al-Zor province might bring the U.S.-backed alliance closer to areas where the Syrian army is operating. But the SDF's member groups have during the conflict mostly avoided confrontation with government forces.

Turkish-backed Syrian rebels, meanwhile, are fighting for control of the northern city of al-Bab, which monitors say is still mostly in Islamic State hands, but which the rebels have pushed into.That battle has brought the Turkish-backed rebels into close proximity with Syrian government forces, which had advanced towards the city from another direction before the rebels entered it.The Syrian army's advance towards al-Bab has raised fears of sparking a confrontation with Turkey.

The United Nations on Tuesday lamented the humanitarian situation around al-Bab.It said an estimated 5,000 civilians were trapped by fighting in the city and surrounding area, and that 300 non-combatants had been killed since December, many of them by air strikes.Turkey and Russia have both been carrying out air strikes around the city."As the operation advances, parties to the conflict may be preparing for urban battles ... which could place civilians in the area at heightened risk of death and injury, as well as (making them) vulnerable to being used as human shields," the U.N.'s humanitarian coordination body said in a statement. (Reporting by Tom Perry, writing by John Davison; Editing by Ralph Boulton and Gareth Jones)

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